The invention relates generally to vehicle drive systems, and more specifically to battery powered drive systems such as those used in battery powered electric vehicles, hybrid electric vehicles, or plug-in hybrid electric vehicles.
Electric vehicles and hybrid electric vehicles are typically powered by one or more energy storage devices, either alone or in combination with an internal combustion engine. In pure electric vehicles, the one or more energy storage devices powers the entire drive system, thereby eliminating the need for an internal combustion engine. Hybrid electric vehicles, on the other hand, include energy storage device power to supplement power supplied by an internal combustion engine, which greatly increases the fuel efficiency of the internal combustion engine and of the vehicle.
Traditionally, the energy storage devices in electric or hybrid electric propulsion systems include batteries, ultracapacitors, flywheels, or a combination of these elements in order to provide sufficient energy to power an electric motor. At start-up of the electric or hybrid electric propulsion system, capacitors within the power electronics or energy storage system of the propulsion system are pre-charged to a prescribed value to allow the electric or hybrid drive to be enabled. This pre-charging function is typically provided using energy stored in an on-board energy storage system, such as a traction battery.
While performing the pre-charging function during vehicle start-up via the vehicle's traction battery is often successful, there are many instances where the traction battery is either non-operational, at a low value of state-of-charge (SOC), or has a terminal voltage below a given threshold. During such instances, the traction battery may be unable to provide sufficient pre-charge energy to the capacitors within the power electronics, and thus the electric drive will not operate, in turn rendering the vehicle inoperable.
One common cause for traction batteries using selected technology to become non-operational is that the batteries are often subjected to ambient temperatures below a specified operating temperature for the battery. That is, when an ambient temperature is below a certain temperature for an extended period of time, the traction battery may cool to a point of solidification. The traction battery may also solidify (or enter a “hibernate” mode) if an electric or hybrid electric vehicle is not operated for an extended period of time (e.g., 12-24 hours) or, in the case of a plug-in electric vehicle, not connected to the power grid within a similar period of time. When a traction battery solidifies, it cannot operate to provide sufficient pre-charge energy to the power electronics of the drive system, and thus the vehicle cannot operate. Unfortunately, many low-cost, high-energy batteries utilized for electric and hybrid electric vehicles, such as high-temperature sodium batteries, are only optimally operable at high temperatures. Additionally, even in instances when an ambient temperature is not below the operating temperature of the traction battery, the battery may still be non-operational based on a low state-of-charge (SOC) or an insufficient terminal voltage. Traction battery operational deficiencies adversely affect the conventional pre-charge function of the power electronics, thereby preventing the electric or hybrid electric vehicle from operating at start-up.
When the traction battery (and, in turn, the vehicle itself) is detected as being non-operational, a fault code may be issued to the vehicle's operating system to aid a technician in determining the cause of the malfunction. It is usually not until the vehicle is transported to a repair facility that such a determination can be made. Accordingly, repairs or adjustments in the field by the vehicle's owner are difficult. Such repairs can be quite costly and inconvenient for the owner, who, at the very least, cannot operate the vehicle until the issue is resolved.
Therefore, it is desirable to provide an electric and/or hybrid electric propulsion system having an alternative mode of providing pre-charge energy to the power electronics of the vehicle in the event that the conventional source of pre-charge energy is non-operational.